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I have a K-12 and have built the Black. This was the first one Suncal designed. I think it is possibly the best sounding. The 4SU is more for tube rollers though it also sounds good. The Black should give the best performance.

I put a passive James-Baxandall tone control circuit in front of my K-12G amp. It was effective, both in tone management AND in causing a dramatic input voltage drop. I removed it promptly.

Now I am thinking of adding an S4 MK III (Black) preamp circuit, followed by the tone circuit, then the K-12G. It looks like the 250v B+ from the amp will be able to drive the 12AU7, as will the 12v heater circuit.

In it I give a final design for the 4S combined with a tone stack. I also give response curves. I would also recommend you look over the chain from about post 20 to post 30 where I talk about the tone stack operation and it's dependence on input and output impedance. There is some very valuable information contained in those posts.

Thanks very much indeed! That work will save me tons of time and effort!

You used 300v for B+ and the original Black circuit used 250v, as does the K-12G. Am I correct in thinking that the reduced B+ will only impact gain? (I'd like to avoid swapping the power transformer if I can.)

The change in B+ has almost no impact on gain. The 4S design works well with B+ voltages from about 240vdc to about 330vdc. The only reason you see 300v there is because I found that the distortion is a little lower with the higher B+ voltage but the design is actually very forgiving.

Just received another 6J6A base headphone amp from Oatley Electroncis in Australia. This time with a toriod power tranni. And again with OPTs. Looks good.

I thought with others saying the 6J6 is cheap and plentiful and the ones in this amp are genuine NOS GEs from 1985, if there shouldn't be a 4S preamp designed around the tube. I guess we need to call on the talents of Matt. I think anytime a tube is cheap and plentiful that this is a good reason to put tube builds in the path of the novice making it cheap and fun. Also us experienced builders could have a some fun to.

So Matt, are you up for the challenge? Does this tube suit well as a preamp tube and does it have any favourable electrical characteristics? A hard question I know but what can we expect from it acoustically? Does it have a good distortion profile? I know you put forward the mixer but what of a proper preamp.

I will post an image in the 6J6 thread also.

Attachment:

image.jpeg

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Ok, I've played with the design a little bit and finally settled on a topology. After a fair amount of fiddling I've decided that you just have to use a single tube per channel. If you don't, you can't get appropriate channel separation and performance. I've also decided that the best way to handle the "off" triode is to run it symmetrically loaded. These two decisions led to this schematic.

Attachment:

6J6 Preamp Schematic.jpg

And for those wondering about the details, here is the load line.

Attachment:

LoadLine.jpg

This will be a relatively "warm" preamp. It should be really good with jazz vocals (e.g. Diana Krall, Norah Jones, etc.). I do have a whole sleeve of these tubes...

Attachment:

6J6 Tubes.jpg

So maybe I should prototype up a couple and give it a try. I'll definately report back if I do.

Well Mark, what do you think?

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Looks good and definitely adheres closely to the 4S principles. Thanks for the quick response.

But humour here. What if you use on tube for both channels. I realise it shares a common cathode and channel separation would suffer. How much would separation suffer though and if we really want to stay true to the 4S rule then we (ok that means you Matt) must do it in one tube.

Think of those times when you have wanted a simple system around the computer or out in the back sunroom. Quiet background music using a minimum of components but all tube.

Also think of those times when you only have one tube of a certain type. I have a box full of one tubers I will possibly never use. Strangely I can't through them out.

Matt, what do you think? Would please put pen to paper and give us a stereo 6J6 preamp. Promise I'll build one and report back. I have one spare tube which came with the latest HP amp from Oatley.

Matt, what do you think? Would please put pen to paper and give us a stereo 6J6 preamp.

Ok. There is a way to do it and maintain channel separation. It is also not recommended by the tube manufacturers.

Now "not recommended" can mean a lot of different things. I'll see if I can make it work for a single ended, common cathode configuration and not burn up the little tube. This may take me a little while to get a design with which I am comfortable.

I just realised I don't have any B7 sockets. Fortunately my interstate parts supplier does. At A$2.45 a piece plus postage I orderd four. Total cost; $17. Ceramic sockets with a nice gold retainer. I know Matt will deliver a unique stereo preamp (hey no pressure Matt) and I want to be ready. I did promise if Matt came up with a single tube version I'd build it.

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